Pyrolysis is the transformation of a compound into one or more other substances by heat alone. In the petroleum and petrochemical industries, pyrolysis is useful for the processing of hydrocarbons. This process is often referred to as “cracking”. When the pyrolysis of hydrocarbons is conducted in the presence of steam, it is often referred to as “steam cracking”. The steam cracking of ethane, propane, naphthas, gas oils and other hydrocarbon feedstocks is a useful process for producing valuable olefins. As a byproduct of the steam cracking process, oxygenated compounds, including carbonyl compounds, are formed. These carbonyl compounds include, but are not limited to, aldehydes and ketones. The amount of carbonyl compounds formed in cracking operations can vary widely, but is typically from about 1 ppm to about 200 ppm in the gas stream with concentrations as high as about 1000 ppm occasionally being encountered because of the use of various feedstocks and cracking temperatures. Byproducts of hydrocarbon cracking processes include the undesirable acid gases CO2 and H2S. Therefore, it is normal for a hydrocarbon cracking plant to have an acid gas removal system to remove CO2 and H2S from the cracked gas. Typically the acid gas removal system usually consists of passing the gas steam through a basic wash (pH greater than 7) to remove acidic components, including hydrogen sulfide and carbon dioxide gas. In the petroleum and petrochemical industries, unit operations involving basic washes are commonly carried out in equipment referred to as ‘caustic scrubbers’ or ‘caustic towers’.
In an acid gas removal system, some oxygenated compounds are also removed. It is known in the art of hydrocarbon processing that certain of these oxygenated compounds, especially carbonyl compounds and particularly acetaldehyde, will undergo polymerization in the presence of the base. When removing carbon dioxide with caustic, aldehydes are trapped. The aldehydes in the caustic solutions reacts producing polyaldols. These polymers known in the industry as Red Oils induce a fouling of the caustic scrubber. In the acid gas removal system, the acetaldehyde polymer will settle on internal equipment surfaces leading to fouling and eventual plugging. Fouling and plugging of the internal equipment means the unit must be shut down to perform cleaning. Every time a unit operation has to be shut down for cleaning it means that a cost is incurred due to lost production, over and above, the actual cost to clean the equipment.
Many prior arts are dealing with such fouling, they essentially describe the introduction of various chemical inhibitors in the caustic scrubber.
During the production of ethylene and propylene with oxygenated feedstock, such as an MTO and alcohol dehydration, aldehydes and carbon dioxide are produced. The amount of aldehydes produced in these processes is very high compared to the steam cracker. The other characteristic of these processes is that very low quantities of aromatics such as benzene are produced. As the concentration of aldehydes is very high, the fouling potential is also very important. In the caustic scrubber operating with the effluent of a steam cracker the presence of aromatics helps to reduce the red oils. On the contrary in the caustic scrubber operating with the effluent of an MTO or alcohol dehydration there are two drawbacks:    (i) there are more aldehydes, as a consequence the red oils are increased,    (ii) there are much less aromatics by produced, as a consequence the red oils are not dissolved and the fouling increased.
It has now been discovered to introduce a solvent, advantageously benzene or toluene or xylenes, in the caustic scrubber and/or in the alkaline solution fed to the scrubber to reduce the formation of the fouling deposits by reducing the Red Oils formation and not only by the dissolution of them.
Prior art has already described introduction of aromatics and/or solvents in a caustic scrubber in the proportion of an inhibitor, it has nothing to see with the proportions and function of the solvent of the present invention. This present solvent reduces the Red Oils formation rate by reducing the contact of growing polymers with Caustic.
U.S. Pat. No. 5,582,808 provides borohydrides that are useful in reducing aldol condensation and subsequent polymer formation in caustic scrubbers. The borohydrides are believed to react with reactive carbonyls yielding more stable alcohols and a salt of the borohydride which remains water soluble, and thus is unlikely to be carried out with the hydrocarbon phase. The borohydrides have the potential to reduce reactive carbonyls at a molar ratio as high as about 4:1::carbonyl:borohydride. A preferred borohydride is sodium borohydride (sodium tetrahydroborate). The borohydride can be introduced in a solvent, toluene may be used but is not desirable (col 3 lines 6-10). Preferably the borohydride is introduced into a caustic solution.
U.S. Pat. No. 5,770,041 describes adding an effective deposit-inhibiting amount of a non-enolizable carbonyl compound to the caustic solution. Preferred non-enolizable carbonyl compounds are formaldehyde, glyoxal, benzaldehyde, p-anisaldehyde, formic acid, glyoxalic acid and paraformaldehyde. The non-enolizable carbonyl compound may be added to the spent caustic wash/stripper system in an amount representing a molar ratio of non-enolizable carbonyl to carbonyl from about 25:1 to about 3:1. Preferably, the ratio is from about 10:1 to about 3:1. Most preferably, the ratio is from about 5:1 to about 3:1. The solution should be added to the system in sufficient quantity to assure that the molar amount of inhibitor is effective to prevent fouling. Treatment ranges of from 1 to 10,000 ppm of inhibitor in the medium may be utilized if no convenient method of measuring carbonyl concentration is available. Where the carbonyl concentration is known or estimable, the inhibitor is preferably added in excess of the carbonyl equivalents. Solvents suitable to dilute the inhibitors include water, . . . pyrolysis gasoline. These two above prior arts don't mention the use of a solvent to reduce the fouling.
U.S. Pat. No. 5,714,055 describes adding an effective depositing-inhibiting amount of a caustic solution-soluble substituted aromatic amine selected from the group consisting of: 2-aminophenol, 4-aminophenol, 4-aminobenzenesulfonic acid and salts thereof, 4-amino-o-cresol, 3-aminophenol, 2-aminobenzoic acid and salts thereof, 3-aminobenzoic acid and salts thereof, and 4-aminobenzoic acid and salts thereof to the caustic solution. A preferred substituted aromatic amine is the sodium salt of 4-amino-benzenesulfonic acid in aqueous solution. The substituted aromatic amine may be added to the alkaline scrubber in an amount representing a molar ratio of amine to carbonyl from about 1.0:10.0 to about 1.0:25.0. Preferably, the substituted aromatic amine may be added to the alkaline scrubber in an amount representing a molar ratio of amine to carbonyl from about 1.0:3.0 to about 1.0:9.0. Most preferably, the substituted aromatic amine may be added to the alkaline scrubber in an amount representing a molar ratio of amine to carbonyl from about 1.0:1.0 to about 1.0:2.0.
U.S. Pat. No. 5,194,143 discloses a method for inhibiting the formation of polymeric based fouling deposits normally formed during the caustic washing of hydrocarbons. The method comprises adding an effective amount for the purpose of an acetoacetate ester compound to the caustic wash system. One mole of the acetoacetate ester compound is needed for every one mole of aldehyde. The acetoacetate ester compound should be added to the caustic wash in an amount from about 0.5 to about 10 moles per mole of aldehyde. Preferably, the feed rate ranges to from 1 about 3 moles of acetoacetate ester compound per mole of aldehyde, with a 1.0 mole ratio being especially preferred. Broadly speaking, from about 1 to about 10,000 parts per million acetoacetate ester compound per million parts basic wash is a sufficient treatment range if no convenient method of measuring carbonyl level is available.